Method of coating stiff materials onto fragile, heat vulnerable substrate webs

ABSTRACT

Stiff materials such as boron and boron carbide are coated onto a polymeric or thin metal heated substrate web, backed by a screen carrier web and passing over an evaporant source of the coating material. The screen is transparent to radiation. The present invention relates to coating of continuous substrate webs with stiff materials.

United States Patent [151 3,660,146 Chadsey, Jr. et a1. May 2, 1972 METHOD OF COATING STIFF [56] References Cited MATERIALS ONTO FRAGILE, HEAT UNITED STATES PATENTS VULNERABLE SUBSTRATE WEBS 2,405,662 8/1946 McManus et al. ..1 17/ 107.1 X [72] Inventors: Earl E. Chadsey, Jr., Sudbury; Frank 2,754,230 7/1956 MCI-earl e! --l l7/1 7-l X Feakes Lexington both of Mass. Averbach ..1 [73 Assignee: National Research Corporation Primary Examiner Alfred L. Leavm [22] Filed: Sept. 29, 1969 Assistant ExaminerKenneth P. Glynn Attorney0liver W, Hayes and Jerry Cohen [211 Appl. No.: 861,585

[57] ABSTRACT R, 117/106 117/1071, Stiff materials such as boron and boron carbide are coated l 1 onto a polymeric or thin metal heated substrate web, backed [51] Int. Cl. ..C23c 11/00, C230 13/00, C23c 13/10 by a screen carrier web and passing over an evaporant source [58] Field of Search ..1 17/106, 107, 107.1; 118/48, ofthe coating material. The screen is transparent to radiation.

The present invention relates to coating of continuous substrate webs with stiff materials.

3 Claims, 6 Dlawing Figures PATENTED MAY 2 I972 [III II v A A A AYAVEA HA A Fig 3.

METHOD OF COATING STIFF MATERIALS ONTO F RAGILE, HEAT VULNERABLE SUBSTRATE WEBS BACKGROUND It is prior invention to produce structural composites by making laminates of coated web substrates. The coatings are stiff reinforcing materials such as boron, boron carbide, aluminum oxide, carbon, and silicon carbide. The substrate is selected from low density materials of very thin web form so that the laminate will have a very high volume fraction of the coating material as high stiffness reinforcement (with minimum parasitic volume and weight of the less stiff reinforcement substrate).

High temperatures are required for coating and under high temperature the substrate web tears or becomes difficult to handle. Lowering temperature of the substrate compromises coating adhesion. Also strength of the deposit is generally a function of deposition temperature. It is desirable that the temperature of the substrate during coating be held at a level which is optimum'for production of high strength coating rather than one determined by the handling characteristics of the substrate;' the former temperature is generally higher than the latter.

It is the object of the invention to provide an improved coatin g process allowing deposition of stiff reinforcing materials at higher temperatures to produce coated webs of reliably higher strength, stiffness and flatness and improved freedom from coating defects and to accomplish this on very thin, low density webs notwithstanding the fragility and heat vulnerability of such substrates.

GENERAL DESCRIPTION An improved coating process is practiced by passing a substrate web over a source of coating vapors with the substrate web backed by a screencarrier web.

The screen carrier may have various forms including a porous wire mesh, a flattened wire mesh, an apertured foil, a solid (i.e. non-porous) foil (including films within the term foil) of infrared (IR) transparent material (e.g. polyolefins, polyimides), a reinforced foil and combinations of the above basic approaches, e.g. an IR transparent foil with actual apertures, a glass fiber or polyimide fiber mesh, a foil reinforced with transparent fiber; an apertured transparent foil reinforced with transparent fiber.

The screen in its various forms has a high thermal transparency, i.e. transparency to infrared radiation to meet the following prior art problem. In a vacuum coating system a large amount of heat reaches the substrate through radiation from the source, particularly in coating stiff materials such as boron wherein the source temperature is raised to 2,000 C. Even after initiating formation of the coating, the heat reaching the substrate can be quite high because boron and similar materials are transparent to radiant energy. The substrates considered herein also tend to have a high degree of thermal transparency. It has been discovered that a thermally opaque carrier web or other opaque backing tends to reflect heat back into the substrate where it is absorbed to raise the substrate to excessively high temperatures, causing degradation of the substrate and of the final coated-substrate product. The carrier web in accordance with the present invention allows a substantial portion of the radiation to be transmitted through it to a controlled temperature heat sink where it is absorbed. It therefore becomes possible to coat at higher rates with less damage to the substrate.

The invention accordingly comprises an improved method of coating stiff materials onto fragile, heat-vulnerable substrates.

Specific embodiments of the invention are now described with reference to the accompanying drawings wherein:

FIGS. 1 and 5 are diagrams of two forms of apparatus for practicing the method of the invention; and

FIGS. 2-4 and 6 are diagrams of several substrate web-carrier combinations utilized in accord with the invention.

Referring now to FIG. l there is shown a vacuum coating chamber 10 to be evacuated by a vacuum pumping system 12 (including the conventional diffusion pump, fore pump, trap, valve and total and partial pressure instrumentation). Within the chamber is a source 14 of coating material heated by means (not shown) such as an induction coil, electron gun or resistance heater to emit vapor stream 16 which condenses on a substrate 18 to be coated. The substrate is moving in the direction indicated by arrow M. A shutter 20 may be interposed between the source and substrate to prevent coating during source warm-up. The substrate is backed by a plate 22 which is cooled by coil 24 carrying a temperature control fluid. A radiant pre-heater 26 is provided for bringing the substrate up in temperature.

The substrate is a web which is unwound from a feed roll 28 and wound up after coating in a finish roll 30 and passes over guide roll 32. Drive motor means (not shown) drive the substrate through the coating process under the necessary tension.

A screen web 34 backs up the substrate and is interleaved with it in the rolls 28 and 30.

Referring now to FIG. 2, one embodiment of screen is indicated as a mesh of wires 234 over the substrate 18. This provides a considerable improvement over no screen in taking up the necessary drive tension and holding the substrate web flat. The screen limits billowing of the substrate to small ridges l7 limited by the weave of the screen.

Another species of the invention is the carrier 234 of FIG. 2 in the form of a flattened screen web. The screen has essentially no weave and the substrate billowing is limited to separate small bumps.

A third embodiment of the invention shown in FIG. 3 involves the use of a perforated sheet 334 as the screen web with hole to hole average distance at least one-fourth of average hole diameter. The holes may be in an ordered array or randomly arranged.

Further, web flatness improvement is obtained in this embodiment.

FIG. 4 shows a wound up substrate web 18 (before coating) interleaved with a screen carrier of the flat sheet type 334 of FIG. 3 or of woven wire type 234 of FIG. 2. The flat type screen has the advantage that the substrate can be placed in an outgassing oven (prior to insertion in coater 10 of FIG. 1 or using the coater as an oven with the source 14 inactive) and then interleavedwith the screen to provide the roll 28 of FIG. 4 with closed ends essentially impervious to gas penetration during storage. On the other hand a web 28 with a woven screen can be outgassed without unwinding the web and enclosed in a container for storage. If a screen is used to carry the substrate web through an unwind-heat-rewind outgassing process, it should be noted whether web stretching takes place during outgassing. lfthis occurs it should be compensated, e.g. by rewinding and interleaving with a separate screen.

FIG. 5 shows another apparatus for practicing the invention in an air-to-air vacuum coating process. The chamber 510 has several sub-chambers separated by seals (not shown) and pumped by appropriate vacuum pumping means 512. The substrate web 18 is unwound and rewound in air. The screen carrier web 534 is an endless belt with drive and tension adjust means 535 outside thevacuum zone. Additional guide rolls may be provided in the vacuum zones as necessary.

FIG. 6 shows a screen web 634 of non-porous form used as a carrier for substrate web 18 in passing over back-up plate 22. The screen is an IR transparent foil such as a polyimide or a polyolefin resin of sufficient thickness (I 2 mils) and strength to serve as a carrier.

It will be apparent from the above disclosure to those skilled in the art that further variations and embodiments can be made within the scope of the invention. Accordingly it is intended that the above disclosure shall be read as illustrative and not in a limiting sense.

What is claimed is:

1. Method of coating a fragile, heat vulnerable substrate web comprising A. supporting the substrate web on a screen carrier which C. heating the substrate web during the coating step of (B) carrier (i) is transparent with respect to radiant heat and to d c i g cragking stresses, f compnses a mes]? dem'mg 531d transparency of 2 The method of claim 1 wherein the screen carrier is of 1) from actual open porosity thereof; flattened mesh form lrarlspomng the Substrate web to be coated and Screen 5 3. The method of claim 1 wherein the screen carrier is an carrier assembled together and unadhered to one another through a coating zone with web'flattening tension applied at least in part to said screen carrier; and

apertured foil.

I! I I l 

2. The method of claim 1 wherein the screen carrier is of flattened mesh form.
 3. The method of claim 1 wherein the screen carrier is an apertured foil. 